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Inorg. Chem. 2000, 39, 4182-4185
Weak Ferromagnetism in a Three-Dimensional Manganese(II) Azido Complex, [Mn(4,4′-bipy)(N3)2]n (bipy ) Bipyridine) Sujin Han,† Jamie L. Manson,‡,§ Jinkwon Kim,*,† and Joel S. Miller*,‡ Department of Chemistry, Kongju National University, 182 Shinkwan, Chungnam 314-701, Korea, and Department of Chemistry, University of Utah, 315 South 1400 East RM 2124, Salt Lake City, Utah 84112-0850 ReceiVed March 21, 2000 Introduction Several families of molecular materials exhibiting spontaneous magnetization below a critical temperature (Tc) have been discovered in the past several years.1 The design of high-Tc ferromagnetic materials has been realized by organizing transition metal ions into three-dimensional networks by use of organic bridging ligands such as cyanide and tetracyanoethenide [TCNE]•-.2,3 Another intriguing synthetic approach to molecular magnets utilizes azide (N3-) as a bridging ligand, due to its coordinative versatility in bridging two transition metal ions.4 Magneto-structural correlations for azido-based magnetic materials have shown that the end-to-end (EE) bridging mode predominantly leads to antiferromagnetic coupling, and the endon (EO) mode to ferromagnetic coupling.4 Examples include dinuclear,5 triangular,6 tetragonal,7 1-D,8 and 2-D9 nickel(II) compounds. * Authors to whom correspondence should be addressed. E-mail:
[email protected] (J.K.);
[email protected] (J.S.M.). † Kongju National University. ‡ University of Utah. § Present address: Chemistry and Materials Science Divisions, Argonne National Laboratory, Argonne, IL 60439. (1) E.g.: Miller, J. S.; Epstein, A. J. Angew. Chem. 1994, 33, 385. Miller, J. S.; Epstein, A. J. Chem. Eng. News, 1995, 73(40), 30. Kinoshita, M. Jpn. J. Appl. Phys. 1994, 33, 5718. Gatteschi, D. AdV. Mater. 1994, 6, 635. Magnetic Molecular Materials; D. Gatteschi, D., Kahn. O., Miller, J. S., Palacio, Eds.; F. NATO ASI Series 198; Kluwer: Dordrecht, 1991. Magnetism: A Supramolecular Function; Kahn, O., Ed.; NATO ASI Series; Kluwer: Dordrecht, 1996. (2) (a) Ferlay, S.; Mallah, T.; Quahas, R.; Veillet, P.; Verdaguer, M. Nature 1995, 378, 701. (b) Hatlevik, Ø.; Buschmann, W. E.; Zhang, J.; Manson, J. L.; Miller, J. S. AdV. Mater. 1999, 11, 914. (c) Holmes, S. D.; Girolami, G. J. Am. Chem. Soc. 1999, 121, 5593. (3) (a) Manriquez, J. M.; Yee, G. T.; McLean, R. S.; Epstein, A. J.; Miller, J. S. Science 1991, 252, 1415. Miller, J. S.; Yee, G. T.; Manriquez, J. M.; Epstein, A. J. In Conjugated Polymers and Related Materials: The Interconnection of Chemical and Electronic Structure; Proceedings of Nobel Symposium NS-81; Oxford University Press: Oxford, 1993; pp 461. (b) Zhang, J.; Zhou, P.; Brinckerhoff, W. B.; Epstein, A. J.; Vazquez, C.; McLean, R. S. Miller, J. S. ACS Symp. Ser. 1996, 644, 311. (4) Corte´s, R.; Lezama, L.; Mautner, F. A.; Rojo, T. In Molecule-Based Magnetic Materials; Turnbull, M. M., Sugimoto, T., Thomson, L. K., Eds.; American Chemical Society: Washington, DC, 1996; pp 187200. (5) (a) Ribas, J.; Monfort, M.; Diaz, C.; Bastos, C.; Solans, X. Inorg. Chem. 1993, 32, 3557. (b) Vicente, R.; Escuer, A.; Ribas, J.; El Fallah, M. S.; Solans, X.; Font-Bardia, M. Inorg. Chem. 1993, 32, 1920. (6) Escuer, A.; Castro, I.; Mautner, F.; El Fallah, M. S.; Vicente, R. Inorg. Chem. 1997, 36, 4633. (7) Ribas, J.; Monfort, M.; Costa, R.; Solans, X. Inorg. Chem. 1993, 32, 695. (8) Hong, C. S.; Do, Y. Angew. Chem., Int. Ed. 1999, 38, 193. (9) Ribas, J.; Monfort, M.; Solans, X.; Drillon, M. Inorg. Chem. 1994, 33, 742.
In an effort to assemble high-dimensional compounds with a greater local spin to enhance the magnetic ordering temperatures, the Mn(II)-azido system has been examined extensively. Escuer and co-workers systematically studied several [MnL2(N3)2]n compounds: 3-D for L ) pyridine10 and 2-D for L ) 3-11 and 4-acetylpyridine,12 methyl-13 and ethylisonicotinate,10 and 4-cyanopyridine.11 Others have also reported highdimensional complexes by use of ancillary multidentate ligands such as bipyrimidine (2-D, EO),14 pyrazine (2-D, EO),151,2bis(4-pyridyl)ethane (2-D, EE),16 and 4,4′-bipyridine (3-D, EE)17 as opposed to monodentate ligands, which promotes additional binding of the metal ions. Composed solely of azido 1,3-bridging units, [NMe4][MnII(N3)3] was found to possess a distorted perovskite-like structure.18 Recent examples, [Ni(5-methylpyrazole)4(N3)][ClO4]8 and [Mn(4-acpy)2(N3)2]n (acpy ) acetylpyridine),11 are weak ferromagnets, whereas all other EE-bonded materials show only antiferromagnetic coupling without a spontaneous moment. A recent communication by Shen et al. reported the synthesis, crystal structure, and preliminary magnetic properties of [Mn(4,4′-bipyridine)(N3)2]n, 1.17 Their magnetic data suggested that 1 did not magnetically order down to 4 K and that only weak antiferromagnetic coupling was present. We found this particularly striking in that many of the known Mn(II)-azido complexes do show interesting magnetic behavior well above 4 K. As part of our studies on transition metal-azido complexes,15 we reexamined the magnetic properties of 3-D Mn(4,4′-bipyridine)(N3)2]n, 1, using detailed magnetic susceptibility and magnetization measurements, and we clearly observe weak ferromagnetism below Tc ) 42.5 K, contrary to a previous report.17 This work emphasizes the importance of taking lowfield magnetic data to characterize the magnetic behavior of materials. Experimental Section Mn(ClO4)2‚6H2O, 4,4′-bipyridine, and NaN3 were uses as purchased (Aldrich). Infrared spectra (400-4000 cm-1) were recorded on a PerkinElmer Spectrum 1000 FT-IR spectrometer with samples prepared as KBr pellets. C, H, and N elemental analysis were performed on an Elemental Anaylsen systeme GmbH Vario EL. Magnetic measurements were taken on a Quantum Design MPMS-5T magnetometer as previously described.19 (10) Escuer, A.; Vicente, R.; Goher, M. A. S.; Mautner, F. A. Inorg. Chem. 1996, 35, 6386. (11) Escuer, A.; Vicente, R.; Mautner, F. A.; Goher, M. A. S. Inorg. Chem. 1997, 36, 3440. (12) Escuer, A.; Vicente, R.; Goher, M. A. S.; Mautner, F. A. Inorg. Chem. 1995, 34, 5707. (13) Escuer, A.; Vicente, R.; Goher, M. A. S.; Mautner, F. A. J. Chem. Soc., Dalton Trans. 1997, 4431. (14) (a) Corte´s, R.; Urtiaga, M. K.; Lezama, L.; Pizarro, J. L.; Arriortua, M. I.; Rojo, T. Inorg. Chem. 1997, 36, 5016. (b) Corte´s, R.; Lezama, L.; Pizarro, J. L.; Arriortua, M. I.; Rojo, T. Angew. Chem., Int. Ed. Engl. 1996, 35, 1810. (15) Manson, J. L.; Arif, A. M.; Miller, J. S. Chem. Commun. (Cambridge) 1999, 1479. (16) Hong, C. S.; Son, S.-G.; Lee, Y. S.; Jun, M.-J.; Do, Y. Inorg. Chem. 1999, 38, 5602. (17) Shen, H.-Y.; Liao, D.-Z.; Jiang, Z.-H.; Yan, S.-P.; Sun, B.-W.; Wang, G.-L.; Yao, X.-K.; Wang, H.-G. Chem. Lett. 1998, 469. The only reported magnetic data were the χT(T) data measured at 4 and 296 K; neither the field at which the data was taken nor the θ value from a fit to the Curie-Weiss equation was reported. The room temperature χT value of 4.27 emu K/mol is substantially greater than the value reported herein. (18) Mautner, F. A.; Corte´s, R.; Lezama, L.; Rojo, T. Angew. Chem., Int. Ed. Engl. 1996, 35, 78.
10.1021/ic000307z CCC: $19.00 © 2000 American Chemical Society Published on Web 08/15/2000
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Inorganic Chemistry, Vol. 39, No. 18, 2000 4183
Figure 1. (a) Molecular structure of 1 showing only the MnII coordination sphere and atom-labeling scheme. (b) Perspective view of the crystal structure of Mn(4,4′-bipy)(N3)2, 1. Shaded, open, and filled spheres represent Mn, N, and C atoms, respectively. [Mn(4,4′-bipyridine)(N3)2]n, 1, was prepared in a way similar to that described in ref 17, by mixing solutions of Mn(ClO4)2‚6H2O, 4,4′bipyridine, and NaN3 in methanol (61% yield). Single crystals suitable for X-ray diffraction study were obtained from slow addition of sodium azide in methanol with a syringe pump during 3 days into the methanol solution of manganese perchlorate and 4,4-bipyridine. Anal. Calcd for C10H8MnN8: C, 40.69; H, 2.73; N, 37.96. Found: C, 40.32; H, 2.71; N, 37.8. IR (KBr, cm-1): ν(C-H), 3059 (w); νas(N3), 2076 (vs); νs(N3), 1323 (vw); ν(4,4′-bipy), 1601 (s), 1411 (s), 815 (s). To verify that 1 was identical to that reported earlier,17 we redetermined the structure; space group ) P41212 (No. 92), M ) 295.18, a ) 8.236(1) Å, c ) 16.937(3) Å, V ) 1148.8(3) Å3, Z ) 4, F(calcd) ) 1.707 g cm-3, R1) 0.0309, wR2 ) 0.0687.
Results and Discussion In light of our findings (vide infra), we verified the crystal structure17 of 1 as it is relevant to the interpretation of the observed magnetic behavior. The molecular structure of 1 showing the coordination environment of the Mn2+ ion is given in Figure 1a, which has slightly elongated octahedral MnII centers composed of four EE-bonded N3- ligands, and two trans-coordinated 4,4′-bipyridine molecules. When extended in 3-D, an unprecedented network structure is formed, Figure 1b. The IR spectrum shows a very strong band at 2076 cm-1 for the νasym(N3) mode, which is consistent with a structure containing an end-to-end bridging ligand. The azide symmetric stretch, νsym(N3), is not active for the symmetrical EE coordination mode. However, 1 has a very weak νsym(N3) band at 1323 cm-1 due to asymmetry of the EE bridges in the complex. A similar result was observed in the [Ni(2,2′-bipy)2(N3)]n(ClO4)n chain complex.21 We verified the crystal structure of 1,17 which has slightly elongated octahedral MnII centers composed of four EE-bonded N3- ligands, and two trans-coordinated 4,4′-bipyridine molecules, which, when extended in 3-D, afford an unprecedented network structure, Figure 1. The tetragonal symmetry (P41212) (19) Brandon, E. J.; Rittenberg, D. K.; Arif, A. M.; Miller, J. S. Inorg. Chem. 1998, 37, 3376. (20) (a) Sheldrick, G. M. SHELX-86 User Guide; Crystallographic Department, University of Go¨ttingen: Go¨ttingen, Germany, 1985. (b) Sheldrick, G. M. SHELX-93 User Guide; Crystallographic Department, University of Go¨ttingen: Go¨ttingen, Germany, 1993. (21) Corte´s, R.; Urtiaga, M. K.; Lezama, L.; Pizarro, J. L.; Gon˜i, A.; Arriortua, M. I.; Rojo, T. Inorg. Chem. 1994, 33, 4009.
Figure 2. χT(T) (O) and reciprocal molar magnetic susceptibility, 1/χ(T) (b), for 1 at 500 Oe.
of the complex precludes a center of inversion. A variety of channel sizes and shapes are produced including triangular, rectangular, and hexagonal. Interestingly, solvent molecules do not intercalate into the cavities nor does a second interpenetrating network form. The 4,4′-bipyridine ligands are not coplanar but are twisted about the C-C bond by 31.9(2)°, and the MnNbipy bond distances are inequivalent [2.256(4) and 2.304(4) Å]. The two kinds of Mn-Nazide bond distances are also different from each other [2.188(4) and 2.208(4) Å], which is consistent with the observation of the symmetric stretching mode in IR. The intranetwork Mn‚‚‚Mn separations are 5.941 (via azide) and 11.647 Å (via 4,4′-bipy). The 2-300 K magnetic susceptibility, χ, of a polycrystalline sample of 1 could be fit to a Curie-Weiss law above 60 K with g ) 2 and θ ) -100 K, indicating strong nearest-neighbor antiferromagnetic interactions, Figure 2. At 300 K, χT(T) has a value of 3.86 emuK/mol and decreases upon cooling due to antiferromagnetic coupling. A sharp increase in χT(T) occurs near 45 K whereby a sudden increase is observed, reaching a maximum value of 6.91 emu K/mol at ∼30 K. This behavior is characteristic of a spontaneous moment due to spin canting. Below ∼30 K, χT(T) decreases abruptly down to 2 K owing to increasing antiferromagnetic interactions and/or saturation effects. Moreover, we and others have observed analogous
4184 Inorganic Chemistry, Vol. 39, No. 18, 2000
Notes
Table 1. Summary of the Structure and Magnetic Properties of Representative Extended Azido Structuresa compounda [Mn2(bpm)(N3)4]n [Mn(pyz)(N3)2]n [Mn(etinc)2(N3)2] [Mn(4-cnpy)2(N3)2]n [Mn(3-acpy)2(N3)2]n [Mn(minc)2(N3)2]n [Mn(4-acpy)2(N3)2]n [Mn(bpe)2(N3)2]n
bridging mode
magnetic behavior
Two-Dimensional (2-D) EO ferro-antiferrod EO ferro-antiferrod EO-EE ferro-antiferrob weak ferroc EO-EE weak ferroc EO-EE weak ferroc EO-EE antiferroc EE antiferrof weak ferroc EE antiferrof
Three-Dimensional (3-D) EE antiferrob weak ferroc antiferrof [Mn(4,4′-bipy)2(N3)2]n EE weak ferroe antiferrof [N(CH3)4][Mn(N3)3]n EE
[Mn(py)2(N3)2]n
J, K
Tc, K ref
g 0.61 g g g g -3.23 -5.52 g -18
g
